1. Field of the Invention
The present invention relates to radio telecommunications and, more particularly, to a packet data telecommunication system for a cellular radio network.
2. Description of the Related Art
In radio telecommunications, such as cellular radio systems, digital modulation schemes, such as time division multiple access (TDMA), are used to transmit both control information and voice traffic over the radio network. In addition, in recent years the transmission of data between computers and other data processing devices over the radio network is increasingly common. One technique which is used for the handling of data traffic over the radio network is circuit switched data services in which a dedicated circuit between transmitting and a receiving station conveys the data from one to the other. An attractive alternative to such circuit switched data services for operators of mobile telephony networks are packet data services. The use of packet data switching enables several mobile users to share the available channel capacity within the system. This technique is well suited to modern data communication applications since data transmissions are usually of a bursty nature and thus do not continuously require a dedicated communications circuit.
A number of different channel access schemes are commonly used in radio communication systems. Each such access scheme has distinct advantages and disadvantages for various applications. For example, fixed assignment access schemes within a radio telecommunications system are used for circuit switched services such as conventional voice telephony and fax. Although not yet widely used in cellular systems, polling schemes may also be employed to enhance the frequency efficiency of a radio system. The most common scheme used for multiple access in a radio system are random access schemes, conventionally employed in many cellular radio telecommunications systems.
In conventional mobile packet radio communication systems, a base station (BS) communicates with a plurality of mobile stations (MSs) over one or more shared packet radio channels. Downlink packet traffic is scheduled by the base station, so that downlink contention between mobile stations is avoided. However, in order for the mobile stations to gain access to the base station on the uplink, they must compete using a random multiple access protocol which inevitably leads to contention and multiple collisions between the different mobile stations which are competing with one another for access on the uplink. Referring to FIG. 1, there is shown a simplified block diagram of a radio communications system which includes facilities for transferring packet data to and from a mobile station. The system 10 includes a communication network 12 which includes a base station/transceiver section 14. The network 12 can be a public land mobile network (PLMN) such as the Personal (formerly, Pacific) Digital Cellular (PDC) system, a digital TDMA cellular radio network.
Network 12 communicates with a mobile station 16 which has the capacity of sending and receiving packet data, via a base station 14 using existing air interface and switching communication protocols. The network 12 also communicates with other mobile stations 20 via a second base station 18 in the network 12, fixed telephones 22 in a public switch telephone network (PSTN), and terminal work stations 24 and 26. As shown, the communication between computer terminal 24 and network 12 are made over a wired line connection. The communication between computer terminal 26 and the network 12 are via a wireless radio connection through base station 14. Consequently, communications to and from phone 22 and computer terminals 24 and 26 can be routed to and from the mobile stations 20 and 16 by means of a network 12.
Referring next to FIG. 2, there is shown the channel structure of an illustrative air interface in a cellular radio system of the type illustrated in FIG. 1 which accommodates random access packet data channel. The channel structure includes a broadcast channel (BCCH) which is used by the network to broadcast various information to mobile stations such as channel allocation and system information. A set of common control channels (CCCH), including a paging channel (PCH) and a single cell signaling channel (SCCH) are used for transmitting signal information. The PCH is used to page a mobile station while the SCCH is used for transmitting information between the network and the mobile units, for example, requests by a mobile seeking access to the network. The uplink channel of the SCCH is of the random access type. A user packet channel (UPCH) is a channel which is available to multiple users for the transmission of user packet data. The uplink channel of the UPCH is also a random access type.
The appended control channels (ACCH) comprise an auxiliary channel appended to the traffic channel (TCH) for transmitting signal information between the network and the mobile station. The ACCH is further divided into the slow appended control channel (SACCH) which comprises a data channel carrying continuous system administration information such as measurement reports from each mobile of received signal strength measurements obtained for both its presently serving cell and adjacent cells. The fast appended control channel (FACCH) is also appended to a TCH and is a channel which temporarily steals the TCH to perform high speed transmissions. A housekeeping channel (RCH) sometimes replaces the SACCH and is used for transmitting maintenance information on the radio channel. Finally, the traffic channel (TCH) is used for transferring encoded speech and circuit switched user data. It is often further divided into full rate TCH and a half rate TCH for encoded speech.
It is conventional today to use the random access method for uplinking data transfer from a mobile station on the user packet channel (UPCH). The channel structure of the cell is communicated to the mobile users within that cell through the information transmitted on the broadcast channel (BCCH). For example, in the PDC system there is broadcast on the BCCH (and on other channels from time to time) a broadcast information message which contains numerous mandatory and optional parameters, including packet channel structure information and channel restriction information. The latter comprises one octet of data of which a small number of the possible 256 bit combinations are used to indicate to the mobile whether or not particular channels are restricted from access by those mobiles.
In accordance with conventional random access procedures, as soon as the user packet data channel (UPCH) is idle, all mobile users which want to send user data packets to the network will simultaneously compete for the use of that channel. If there is only one access during this competition phase, that user will get hold of the channel and remain its user until the complete data packet has been sent. During the time when the user utilizes the channel, no other mobile seeking to transmit a data packet will try to access it. However, if during the competition phase there is more than one user which simultaneously accesses the channel, a collision occurs and a maximum of one, or often none of those competing users, will get data through the channel. In such cases, each failing user must wait a random time period before it can make a new attempt to seize the channel.
The use of shared random access data channels in conventional packet services within radio networks has numerous disadvantages. For example, during high traffic loads and long packet messages, the probability of a mobile station being able to send its data packets is dramatically reduced and a mobile must wait an inordinately long period of time for the channel to become free so that it can even attempt to access it.
As illustrated in FIG. 3, each of the two mobile stations 31 and 32, equipped respectively for handling packet data from two portable computers 31a and 32a receive information broadcast on the downlink of the air interface, 33 and 34 respectively. Each mobile 31 and 32 receives the same information 35 broadcast on the BCCH. If both of the mobile stations 31 and 32 seek to send packet data to the network, they both listen for information on the BCCH indicating the availability of a random access user data channel (UPCH). An algorithm which uses the mobile's own unique identity (MSI) as one input parameter attempts to spread the mobiles evenly over the available channels. We assume each of the two mobiles 31 and 32 find the same UPCH 38 when applying the algorithm. If their respective access data packets 36 and 37 do not collide and obliterate one another when received at the base station, the packets 36 and 37 are successfully delivered to the network. If, instead, two user packets 36 and 37 collide, then its likely that neither of the two mobile stations 31 or 32 succeeds to access the channel and both must wait a random period of time before it make a new attempt to access the channel. The random access control process in a digital mobile radio communication system of the PDC type illustrated in FIGS. 1 and 3, is shown in FIG. 5.
Once a mobile successfully has started sending a packet it will continue to complete that packet. Each packet transfer is done under competition with other mobiles. FIG. 4 illustrates the layer 1 view of an uplink access scheme if we assume MS1 "has" the channel.
It is obvious that the more MSs that the algorithm allots to the same UPCH, the higher the risk of colliding packets.
In FIG. 5, the downlink user packet channel UPCH, and signaling channel SCCH, include a collision control field 41. This field is labeled E and, in this example, is 22 bits in length. This information is used by the mobile station during random access. Processing of the collision control bit field E at the base station comprises the processing of several subfields including the setting of an I/B field 42 to the bits "111" if the uplink UPCH is idle and to "000" if the uplink UPCH is busy. An R/N field 43 is set by the base station to "111" if valid information was received on the UPCH channel in the previous slot and to "000" if no valid information was received on the UPCH channel in the previous slot. The PE field 44 is set to all zeros if the channel is idle or no message was received. If a message is received on the UPCH channel, the detected and checked (CRC) (16 bits) from the UPCH message received from the mobile station are used as a partial echo in the PE field 44 in the downlink transmission.
With respect to processing of the packet data information in the mobile station, when the mobile station has data to send, it sequentially checks UPCH channels for an idle condition and starts the transmission. Next it looks for the R/N and PE fields to confirm that the first packet unit was correctly received by the base station. If this did not occur, the mobile station will, after a random delay, look again for an idle UPCH channel and try to retransmit its packet.
Referring next to FIG. 6, an example of random access control between two mobile stations in an illustrative digital cellular system of the PDC type is illustrated. In this example, two mobile stations MS1 and MS2 each have a packet to transmit to the network. The packets both consist of two bursts on the UPCH channel. The sequence of events corresponds to the sequence of circled numbers in FIG. 6. First, the uplink UPCH is idle, which is indicated by the E field on the downlink UPCH, and thus both mobiles start transmission of their packets. Second, the base station is able to receive the first packet burst from MS2 uncorrupted and responds accordingly by setting the following indications in the E field on the downlink: I/B field: B=B (busy); and R/N field: =R (burst received); and PE field: the CRC value from the burst received from MS2. Thirdly, MS2 detects that the PE field contains the CRC from the burst it has transmitted, which together with the appropriate B and R indications tell this mobile station to continue transmitting its packet. MS1, since it lost the contention with MS2, will inhibit all transmissions for a random time and then start searching for an indication that the channel has become idle again. In the fourth step, when mobile station MS2 has completed its transmission the channel will again be marked idle and, in this example, MS1 starts transmission of its packet. At 5, MS1 receives an indication that its first burst was correctly received.
From these illustrations, it can be seen how a mobile station, seeking random access within the system could encounter substantial difficulty in obtaining use of the packet data channel when either a great deal of packet traffic is present in the network or the packets being sent by the packet channel user are lengthy and therefore occupy the channel for extended periods of time.
Thus, there exists a need for an alternative solution within such radio telecommunication networks which enhance the packet data access by users within the system.